Process for preparing polyalcohols from formaldehyde having a low formic acid content

Abstract

The invention relates to a process for preparing polymethylol compounds of the formula (I) where the radicals R are each, independently of one another, a further methylol group or an alkyl group having from 1 to 22 carbon atoms or an aryl or aralkyl group having from 6 to 22 carbon atoms, by condensation of aldehydes having from 2 to 24 carbon atoms with formaldehyde in an aldol reaction using tertiary amines as catalyst to form alkanals of the formula (II) where the radicals R each independently have one of the abovementioned meanings, and subsequent hydrogenation of the latter. The particular inventive feature of this process is that the aldol reaction is carried out using an aqueous formaldehyde solution having a formic acid content of <150 ppm and preferably <100 ppm. In this way of carrying out the process, the formation of by-products can advantageously be prevented in a targeted manner and the yield of the desired polymethylol compound can thereby be increased.

1. A process for preparing polymethylol compounds of formula (I) 
       
         
           
           
               
               
           
         
         comprising: 
         condensing an aldehyde having from 2 to 24 carbon atoms with formaldehyde in an aldol reaction to form an alkanal of formula (II); 
       
       
         
           
           
               
               
           
         
       
       and
 hydrogenating the alkanal of formula (II); wherein 
 the radicals R are each, independently of one another, a further methylol group or an alkyl group having from 1 to 22 carbon atoms or an aryl or aralkyl group having from 6 to 22 carbon atoms, 
 the condensing an aldehyde with formaldehyde is catalyzed by a tertiary amine, and 
 the formaldehyde is an aqueous formaldehyde solution having a formic acid content of <150 ppm. 
 
     
     
       2. The process according to  claim 1 , wherein the formic acid content of the aqueous formaldehyde solution is <100 ppm. 
     
     
       3. The process according to  claim 1 , wherein the alkanal of formula (II) is one selected from the group consisting of propionaldehyde, n-butyraldehyde, acetaldehyde and isobutyraldehyde and the prepared polymethylol compound of formula (I) is correspondingly trimethylolethane, trimethylolpropane, pentaerythritol or neopentyl glycol. 
     
     
       4. The process according to  claim 1 , wherein the condensing and hydrogenating are carried out continuously. 
     
     
       5. The process according to  claim 1 , wherein the tertiary amine catalyst is present in such an amount that a pH of from 5 to 12 is established. 
     
     
       6. The process according to  claim 1 , wherein the tertiary amine is trimethylamine. 
     
     
       7. The process according to  claim 1 , wherein a molar ratio of alkanal to formaldehyde is in the range 1:2 to 1:5. 
     
     
       8. The process according to  claim 7 , wherein the molar ratio of alkanal to formaldehyde is in the range 1:2 to 1:3.5. 
     
     
       9. The process according to  claim 1 , wherein 0.001 to 0.2 equivalents of tertiary amine, based on alkanal, are present in the condensing of an alkanal with formaldehyde. 
     
     
       10. The process according to  claim 9 , wherein 0.01 to 0.07 equivalents of tertiary amine, based on alkanal, are present in the condensing of an alkanal with formaldehyde. 
     
     
       11. The process according to  claim 1 , further comprising treating the formaldehyde with a basic ion exchanger prior to use in the condensing of an alkanal with formaldehyde. 
     
     
       12. The process according to  claim 2 , wherein the formic acid content of the aqueous formaldehyde solution is <50 ppm. 
     
     
       13. The process according to  claim 1 , further comprising distilling a reaction mixture obtained by the condensing of an alkanal with formaldehyde, wherein
 a low boiling fraction comprising unreacted alkanal and formaldehyde is recycled to the condensing of an alkanal with formaldehyde. 
 
     
     
       14. The process according to  claim 1 , wherein a solvent or solubilizer is added. 
     
     
       15. The process according to  claim 14 , wherein the solvent or solubilizer is at least one selected from the group consisting of tetrahydrofuran, dioxane, methyl tert-butyl ether, methanol, ethanol and 2-ethylhexanol. 
     
     
       16. The process according to  claim 1 , wherein a temperature of the condensing of an alkanal with formaldehyde is in the range from 5 to 100° C. 
     
     
       17. The process according to  claim 16 , wherein the temperature of the condensing of an alkanal with formaldehyde is in the range from 15 to 80° C. 
     
     
       18. The process according to  claim 1 , wherein a temperature of the hydrogenating is in the range from 50 to 180° C. 
     
     
       19. The process according to  claim 18 , wherein the temperature of the hydrogenating is in the range from 90 to 140° C. 
     
     
       20. The process according to  claim 1 , wherein a pressure of the hydrogenating is in the range from 10 to 250 bar.